From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-72416][MODERATE REGULAR] netfilter: nft_compat: ebtables emulation must reject non-bridge targets [ Upstream Date: Sat, 15 Aug 2026 19:53:44 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-72416 X-AL-KERNEL-Priority: MODERATE REGULAR X-AL-KERNEL-Severity: MODERATE REGULAR X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 4 X-AL-KERNEL-ActionableScore-Lower: 1 X-AL-KERNEL-Commit: efc17b9240d821c424bc5191a5c6e9384a06293e List-Id: CVE: CVE-2026-72416 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: netfilter: nft_compat: ebtables emulation must reject non-bridge targets [ Upstream Commit: efc17b9240d821c424bc5191a5c6e9384a06293e Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=efc17b9240d821c424bc5191a5c6e9384a06293e Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72416 Analysis date: Sat, 15 Aug 2026 19:53:44 -0400 ActionableScore: 4 ActionableScore lower bound: 1 Actionable bucket: Borderline manual review recommended / Actionable only for delegated CAP_NET_ADMIN or firewall policy boundary deployments Manual review required: YES Summary: nft_compat could allow non-bridge xtables targets in bridge rules, causing incompatible verdict handling and possible firewall policy bypass when an attacker can install affected nftables bridge rules. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-72416 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72416 The original announcement does not normally provide a security severity estimate, CVSS assessment, or enough information to determine whether the reported kernel bug represents a practically relevant security issue. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. It combines an autonomous classifier with LLM-assisted technical analysis and a separate ActionableScore mechanism. The purpose of this report is to prioritize Linux kernel CVEs before manual review, identify cases that require prompt investigation, and support automatic closure of issues that are unlikely to have meaningful security impact. Published priority for this report: MODERATE REGULAR Manual review required: YES A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-72416 MODERATE CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N';*CWE-20;CWE-693;CWE-284;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '5.5';DESCR 'nft_compat for the bridge family could accept a non bridge xtables target through NFPROTO_UNSPEC fallback even though xtables and ebtables use incompatible verdict values. This can cause bridge packet filtering rules to evaluate with unexpected verdict semantics and may bypass the intended firewall policy. For the CVSS the PR:L is used for the paranoid interpretation because CAP_NET_ADMIN can be delegated to container root, reduced capability service accounts, or network management components that are not full host administrators. The issue is not directly network reachable without a vulnerable ruleset being installed first. Impact is mainly integrity loss through firewall policy bypass rather than kernel memory corruption or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Actionable only for delegated CAP_NET_ADMIN or firewall policy boundary deployments (with actual score 3) YES REMOTE NETFILTER DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=4 ActionableScoreLower=1 ## 1. ActionableScore * Conservative score: 1 * Paranoid score: 4 * Final recommended bucket: **Borderline manual review recommended / Actionable only for delegated CAP_NET_ADMIN or firewall policy boundary deployments**:: ## 2. Signal breakdown Conservative signals: * Integrity impact plausible: +1. Incompatible NF verdict vs EBT verdict semantics can cause bridge filtering policy to behave differently than intended. * Common networking / packet-filtering path: +1. The impact is in netfilter bridge packet filtering once such a rule exists. * Requires CAP_NET_ADMIN in typical deployments: -2. Creating nftables compat bridge rules normally requires administrative network privileges. Conservative total: 1. Paranoid additional interpretation: * Cross-boundary policy bypass: +1. If rule management is delegated to a container root, network service account, or restricted firewall manager, the bug can bypass the intended bridge firewall policy model. * Broad security relevance of netfilter policy path: +1. Netfilter rules are commonly used as an enforcement boundary even though this specific nft_compat bridge path is configuration-dependent. Paranoid total: 4. No memory corruption, UAF, OOB write, refcount abuse, or crash primitive is shown by the patch. ## 3. Reachability analysis The bug is triggered through nftables rule installation in the bridge family using nft_compat targets. In normal host deployments this requires CAP_NET_ADMIN, so it is not reachable by a plain unprivileged local user. Namespaces and containers matter because CAP_NET_ADMIN may be delegated to container root or network-management services that are not equivalent to full host root. The issue is not directly network reachable by packets alone. Network traffic can only observe or benefit from the policy error after a vulnerable ruleset has been installed. Call-site confidence: high. The patch directly changes validation selection for NFPROTO_BRIDGE targets and prevents NFPROTO_UNSPEC fallback. ## 4. Severity interpretation This behaves like a netfilter policy validation bug, not a memory corruption vulnerability. Realistic impact is integrity loss through firewall or bridge filtering policy bypass. It is not an Important-class kernel memory safety issue by itself. It becomes more actionable when affected systems delegate nftables management or rely on bridge nftables rules as a security boundary. ## 5. One-sentence report phrase nft_compat could allow non-bridge xtables targets in bridge rules, causing incompatible verdict handling and possible firewall policy bypass when an attacker can install affected nftables bridge rules. ## 6. Manual review recommendation MANUAL CHECK REQUIRED. Reason: this is not memory corruption, but it is a netfilter policy bypass candidate where final severity depends heavily on whether CAP_NET_ADMIN or firewall rule management is delegated to reduced-privilege users, containers, or automation. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: netfilter: nft_compat: ebtables emulation must reject non-bridge targets [ Upstream Commit: efc17b9240d821c424bc5191a5c6e9384a06293e Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=efc17b9240d821c424bc5191a5c6e9384a06293e Changed files: net/netfilter/nft_compat.c Diff excerpt: Not included in this email. See the upstream URL for the full patch. Full patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=efc17b9240d821c424bc5191a5c6e9384a06293e ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-72416 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72416 The original CVE announcement normally does not include a security-level estimate. In particular, it may not contain a CVSS assessment, an impact level, or enough information to determine whether the reported bug is a practically relevant security issue. One purpose of this parallel CVE list is to provide that missing technical and prioritization information. The original goal of the AL-KERNEL project was to prioritize Linux kernel CVE analysis automatically before manual review. The system can also help identify non-security issues that may be suitable for automatic closure. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. The first analysis stage combines an autonomous classifier with additional LLM-based analysis. The autonomous classifier runs locally on a CPU and is based on a backpropagation neural network. Together, these mechanisms produce a technical vulnerability description, identify likely weakness types, estimate CVSS severity, and provide input for ActionableScore. Two CVSS estimates are retained because incomplete kernel vulnerability information often permits more than one defensible interpretation: Conservative CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:N The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N The Best / paranoid CVSS score: 5.5 The conservative vector represents a lower-impact interpretation. The Best/paranoid vector intentionally represents a plausible upper-bound interpretation and should not automatically be treated as demonstrated real-world impact. CVSS may also need to be adjusted for a particular Linux deployment, because actual reachability, privileges, enabled kernel configuration, hardware, namespaces, exposed device nodes, and other environmental conditions can differ significantly between systems. A separate ActionableScore mechanism evaluates practical remediation urgency. Its analysis may include reachability, attack prerequisites, subsystem exposure, memory-corruption characteristics, denial-of-service reliability, and possible confidentiality, integrity, or privilege-escalation impact. Conservative ActionableScore: 1 Paranoid ActionableScore: 4 The final base severity is taken directly from the second tab-separated field of the AL-KERNEL classification result. ActionableScore does not replace or independently override that final AL-KERNEL decision, and if ActionableScore adjusted impact level of ALKERNEL, then you would see self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7. For an AL-KERNEL result of MODERATE, this report uses the following additional presentation split: ActionableScore below 5 -> MODERATE REGULAR ActionableScore 5 or more -> MODERATE 7.0 The distinction between MODERATE REGULAR and MODERATE 7.0 makes it possible to identify Moderate issues that should receive manual analysis and fixes before lower-priority MODERATE REGULAR issues. In many cases, MODERATE REGULAR fixes may wait for a later rebase or routine update. There is one override in which MODERATE REGULAR becomes MODERATE 7.0 even when the ActionableScore is below 5. When the AL-KERNEL result contains the KPANIC flag, a MODERATE result is always presented as MODERATE 7.0. The KPANIC flag selected with few regexps without usage of AI at all, so it helps to detect cases when Kernel Crash happens and similar (to filter False-Negative results from the LLM usage). KPANIC indicates that a reliable kernel crash, kernel panic, or similarly serious kernel availability impact was identified by the classification workflow. AL-KERNEL base severity for this report: MODERATE KPANIC detected for this report: NO Published priority for this report (same as in Subject): MODERATE REGULAR These results are intended to support engineering triage. They are machine-generated estimates, and cases marked for manual review should be validated by a human security engineer before final disposition. For more info read docs linked from here: https://kernelcve.org/ (and you can submit you own patch there to generate such a report for non-existant CVE-id yet). Note that in many cases this AI tool selects higher severity, than real is (means you can expect Importants instead of Moderate 7.0 or Moderates 7.0 instead of regular Moderates). If you see such cases, please use reply email interface to add additional manual analyses info to this particular CVE. And please, please, let me know when you see Lows instead of Importants or Important instead of Low (because particular for such cases I need to tune this AI tool to make it better for this one and next similar). My contact email for such notifications is alexanjelausa@gmail.com (and both send reply to CVE record itself too and see "reply" button below for howto reply).